Electrochromic glazings are coated glass laminate structures wherein light transmission through the laminate structure is modified by application of an electrical potential difference across the glazing. Glazings of this type are often used, for example, in motor vehicles, particularly as sun roofs, since their use permits a degree of control over the amount of solar energy entering the vehicle's passenger compartment.
Such glazings typically comprise a first ("top" or "upper") and a second ("bottom" or "lower") glass support sheet which define the outer surfaces of the glazing. Coated upon an inner (with respect to the laminate) surface of each support sheet is a transparent electroconductive layer. These coated glass support sheets are further successively separated by: 1) a layer of an electrochromic material, which functions as an electrode, 2) an electrolyte layer and; 3) a counterelectrode layer. The counterelectrode layer may, if desired, be formed from an electrochromic material. An electrical power lead is inserted into each of the electroconductive layers.
Thus, upon the application of an electric current to the system, an electrical field is created, resulting in the reversible insertion of cations (provided by the electrolyte layer) into the electrochromic layer. As a result, an electrochemical reaction occurs within the electrochromic material, wherein this material shifts from a first oxidative state to a different oxidative state. As shown below, this oxidation state change induces a corresponding color change within the electrochromic material. For example, when tungsten oxide is used as the electrochromic material, the reversible reaction proceeds as follows: ##STR1## wherein M.sup.+ =H.sup.+, Li.sup.+, Na.sup.+, K.sup.+.
Although, as noted above, the electrolyte layer is the source of the cations necessary for the reaction, it is the counterelectrode layer which permits the M.sup.+ cations to move back and forth between the electrolyte and electrochromic layers within the glazing.
In forming electrochromic glazings of the type described above by the methods used in the prior art, a first glass support sheet is coated upon at least a portion of its inner surface, i.e., the side facing the interior of the laminate, with a transparent electroconductive layer and a layer of an electrochromic material. A second glass support sheet is also coated on an inner portion thereof with a transparent electroconductive layer, as well as with a counterelectrode layer. An electrolyte layer is then coated either upon the electrochromic material or the counterelectrode layer on the first or second sheets, respectively. Thereafter, the stacked layers coated onto the two glass sheets are sandwiched together into a laminate such that the uncoated portions of the first and second glass support sheets define the outer, i.e., top and bottom, surfaces of the glazing.
As shown for example, in European Patent Publication No. 338 876, the prior art assembly operation described above is typically carried out by positioning the two coated support plates such that the coated layers abut against one another, then heating the layer stack under pressure at a sufficiently high temperature so that the electrolyte, which has an ion conductive polymer base, becomes adherent The pressure used ranges, for example, between 5.multidot.10.sup.5 Pa and 6.multidot.10.sup.5 Pa at a temperature of 85.degree. C. Thereafter, the two coated support plates are sandwiched together to form the laminated glazing. Further, upon completion of this heat treatment, a seal is placed around the periphery of the glazing to prevent contamination of the electrolyte layer by water vapor from the surrounding atmosphere.
One major drawback to the use of this prior art process however, is that it results in the formation of bubbles visible to the naked eye within the glazing. These bubbles are often not initially seen but rather, they gradually appear over time and are particularly visible in the colored phase of the electrochromic material. This effect has been further demonstrated by aging tests wherein the formation of the bubbles is accelerated by subjecting glazings of the type described above to elevated temperatures, such as might occur with an automobile sunroof exposed to the rays of the sun. Such bubbles are clearly undesirable in that they have an obviously negative aesthetic effect.
It has been determined by an analysis of these bubbles that their composition is very close to that of the air in the zone wherein the glazing is formed. The source of these bubbles is therefore believed to be that air which becomes trapped in the electrochromic system during the assembly process. The trapped air remains invisible at first because it is in the form of microbubbles. However, as time passes and the glazing is exposed to elevated temperatures, e.g., such as caused by the rays of the sun striking the surface of the glazing, these microbubbles coalesce and become visible, thus marring the appearance of the entire glazing.
It has therefore been desired for some time by those working in this field to develop a method and apparatus for forming electrochromic glazings of the type described above which prevents the formation of bubbles therein and thus to produce a glazing whose surface is not marred by such bubbles.